A need presently exists for an electrically-conductive additive for paints, plastics, papers, and similar products which (a) will provide desirable electrostatic discharge and electromagnetic shielding properties, (b) will provide long service life, (c) is inexpensive to make and use, and (d) will enable the achievement of desired colors and/or transparency.
Carbon black, various metals, certain organic amines and amides, and doped tin oxide have been used heretofore as additives in paints, plastics, and paper products to provide desirable electrostatic discharge and/or electromagnetic shielding properties. However, these additives have significant shortcomings. Carbon black and the various metal additives used heretofore generally hinder and/or prevent the attainment of certain desirable colors and/or transparency. Products containing carbon black are also susceptible to sloughing. Organic amine and organic amide additives, on the other hand, generally have undesirably high solubilities and volatilities. Thus, products containing amine or amide additives typically have short service lives, low durabilities, and poor weather and humidity resistance characteristics. Finally, although doped tin oxide is desirably light-colored and electrically conductive, doped tin oxide is costly to produce.
Titanium dioxide is a wide band gap semiconductor which can be made conductive by doping with Group IIIA metal oxides. For example, U.S. Pat. Nos. 5,124,180 and 4,990,286 disclose a chemical vapor deposition (CVD) process for coating a substrate surface with a transparent, electrically-conductive film. The CVD process of U.S. Pat. No. 5,124,180 and 4,990,286 can be used to produce liquid crystal display devices, solar cells, electrochromic absorbers and reflectors, energy conserving heat mirrors, and antistatic coatings.
Unfortunately, conductive films such as those produced by the CVD process of U.S. Pat. Nos. 5,124,180 and 4,990,286 are susceptible to sloughing, scratching, and abrasion. Further, due to the difficulty and expense involved in coating large items and items having numerous and/or intricate surfaces, CVD processes do not provide a practical means for producing conductive plastic articles. Moreover, the type produced by the CVD process of U.S. Pat. Nos. 5,124,180 and 4,990,286 are not obtained in, and cannot be readily converted to, particulate forms which are suitable for addition to paints, plastics, papers, and other such products.
Other factors also mitigate against the commercial use of CVD-type processes. Due to the highly reactive nature of the compounds required for use in CVD-type processes, the use of such processes on a commercial scale would be quite dangerous. The use of CVD-type processes on a commercial scale would also not be cost effective since (1) the volatile compounds used in these processes are expensive and (2) only a fraction of the compound used in a CVD-type process actually reacts and deposits on the substrate. The requirement that heated substrates be used also reduces the commercial viability of CVD-type processes.
As is well known in the art, titanium oxides are produced using vapor phase oxidizing processes. Vapor phase oxidizing processes used for producing particulate titanium oxide products are generally discussed, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, John Wiley and Sons, 1982, Volume 17, the entire disclosure of which is incorporated herein by reference.
U.S. Pat. No. 3,022,186 describes the production of titanium dioxide solid solutions using a wide variety of compounds. The patent is directed to the use of metal fluorides of magnesium, zinc, manganese(II), iron(II), cobalt(II), nickel(II) and palladium(II) to dope the lattice.
U.S. Pat. No. 3,794,508 does not relate to titanium dioxide production but describes production of a fluoride-doped metal titanate to produce a fibrous alkali metal titanate.
U.S. Pat. No. 3,821,359 discloses the use of hydrofluoric acid to dissolve titanium dioxide in order to change pigment optical properties. It is not related to doped titanium dioxide.
U.S. Pat. No. 3,859,109 describes the precipitation of coatings containing the oxides of zirconium, hafnium or titanium on titanium dioxide pigments. It does not disclose production of a fluoride-doped titania.
U.S. Pat. No. 3,956,006 discloses the use of polyvalent metal fluorides which act as habit modifiers in growth of potassium hexatitanate.
U.S. Pat. No. 4,058,393 relates to a process for recovering titanium dioxide from ores. Impure titanium dioxide is dissolved in a solution which contains fluorides. Upon precipitation, titania and titanium oxyfluoride are precipitated. The oxyfluoride is decomposed either by a thermal treatment or by reaction with steam to yield a titanium dioxide product which is essentially fluoride free.
U.S. Pat. No. 4, 168,986 discloses the use of fluoride salts, such as sodium fluoride, as a substrate release material used in the preparation of lamellar pigments.
U.S. Pat. No. 4,780,302 describes a process for production of a metal fluorotitanate which is a distinctly different compound from a fluorine doped titania.
U.S. Pat. No. 5,118,352 is directed to a process for deposition of colloidal titanium dioxide on flakes of a supporting material. The supporting material can be mica or a similar substance such as fluorophologopite.
U.S. Pat. No. 5,173,386 discloses a material which is electrophotographic, i.e., the material is conductive only when illuminated. Such materials are different from fluorine doped titania which is an electronic conductor and is conductive both in the presence and absence of illumination.
In addition, the material in the '386 patent does not contain a fluoride ion after the preparation process is complete.
Japanese Patent Kokoku 63-35977 describes a product which consists of titanium dioxide with fluoride ion adsorbed on the surface of the solid. The adsorbed fluoride ion is then removed by high-temperature treatment followed by washing.
Japanese Patent 88035977 discloses an electrophotographic material, i.e., one that is conductive only in the presence of illumination by light. Additionally, this patent only describes surface doping by reaction of titanium dioxide particles with aqueous fluoride solutions.
U.K. Specification 1,442,756 refers to the application of fluorinated organic compounds to the titanium dioxide surface to reduce agglomeration of pigment during transportation and storage.
U.K. Application 2,115,394 describes the application of alumina surface coatings from an aqueous solution containing, among other ions, fluoride ion.
U.K. Application 2,161,494 relates to a process for precipitation of a pigment from an acidic or basic solution containing fluoride ion. The application does not disclose the production of a titania pigment.
The coating processes of the prior art and the products produced thereby have numerous undesirable characteristics. The oxide products produced are only surface coated. Thus, substantial conductivity losses are realized as surface attrition of the particulate product occurs. Additionally, at least two entirely separate processes are required to complete the coating methods. In the first process, a particulate product is produced and recovered. In the second process, the particulate material is fluorine treated and heat treated. Further, in comparison to the inventive process described hereinbelow, the coating processes of the prior art are very slow.